Alkali-metal hexamethyldisilazanes have been prepared from hexamethyldisilazane (HN[Si(CH.sub.3).sub.3 ].sub.2 or HMDS) under a number of synthetic schemes. However, each of these synthetic schemes has one or more significant drawbacks associated therewith.
Under one such synthetic scheme, alkali-metal hexamethyldisilazanes are prepared using organometallic reagents such as n-butyllithium, phenyllithium, phenylsodium and phenylpotassium. An example of such a reaction using n-butyllithium to produce lithium hexamethyldisilazane is provided below: EQU n-BuLi+HN[Si(CH.sub.3).sub.3 ].sub.2 .fwdarw.LiN[Si(CH.sub.3).sub.3 ].sub.2 +CH.sub.3 CH.sub.2 CH.sub.2 CH.sub.3 .uparw.
Such organometallic reagents are generally pyrophoric, however, and thus create tremendous material handling problems.
Alkali-metal hexamethyldisilazanes are also prepared using alkali-metal amides or alkali-metal hydride reagents at room temperature as shown below: EQU LiH+HN[Si(CH.sub.3).sub.3 ].sub.2 .fwdarw.LiN[Si(CH.sub.3).sub.3 ].sub.2 +H.sub.2 .uparw. EQU LiNH.sub.2 +HN[Si(CH.sub.3).sub.3 ].sub.2 LiN[Si(CH.sub.3).sub.3 ].sub.2 +NH.sub.3 .uparw.
Reactions of alkali-metal amides or alkali-metal hydride and hexamethyldisilazane generally do not proceed to completion, however, and result in the formation of entrapped, unreacted amides or hydrides which can give rise to undesirable by-products and safe-handling problems.
Lithium diisopropylamide (LDA) has been produced directly from lithium metal using an electron carrier such as styrene or isoprene in THF as follows: ##STR1## See U.S. Pat. No. 4,595,779. Suitable electron carriers are conjugated, unsaturated hydrocarbons. These electron carriers are noted to readily accept electrons from an alkali metal to form free radicals or carbanions.
However, the use of electron carriers such as styrene in such reactions usually results in formation of toxic alkylbenzene side products and necessitates difficult separation of the product.
It is very desirable to develop a synthetic scheme for the production of alkali-metal hexamethyldisilazanes that does not suffer from the drawbacks associated with the above synthetic schemes.